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Old 2005-02-18, 01:32 AM
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Re: A question about speed correction of lossless files

I need to shave 50ms off of a separate WAV file to wild sync it with a 12 minute cartoon from pro digital tape. ugh! This is just temporary until we get a SMPTE system wired up. I hope foobar has fine enough controls.

Well, here's the story of pitch shifting, as I know it:

Tape decks always have a pitch control (usually buried somewhere in the circuit board, sometimes can be controlled from the front panel). For years this was the only pitch shifting available. Dead simple, it's a control that adjusts the speed of the motor. Playback at a faster speed, pitch is also increased. Play back at a slower speed, pitch is decreased. Also of importance is that time (& thus tempo) are made shorter or longer sympathtically.

On an analog multitrack machine, all of the tracks run at the same speed. So to affect the pitch of one track means also changing the pich of the other tracks (equally). So, to pitch shift an overdub up a bit the multitrack would have to be run at a slightly slower speed during layering, or vice-versa for pitch shifting down. I'm sure some other more labor-intense strategies were used as well.

Then sometime in the 70's an outrageous new invention called the "Pitch Shifter" was invented. It could digitally alter pitch without altering time with just a tiny lag for processing time. Prices were incredibly high, it still costs around $5000 for an Evantide Ultra-Harmonizer.

Now we have all this and more in software.

So, in the case of a tape of a show, when you can hear the speed is all wrong, it means your deck isn't playing at the same speed the original tape was recorded on. Ideally and by far the best option is to adjust the pitch of your analog deck right at the stage the tape is being played into the digital recorder.

If this is impractical, digital speed correction is necessary.

Preserving tempo is useful for pitch correction of a single wrong note (on a discrete track).

Preserving pitch is useful for a television documentary where you want to adjust spoken passages to the exact right lengths to sync with the timing of the film. In this case you want the pich of the announcer's voice to stay the same and the tempo (and therefore length of time) will be stretched or squashed.

Since we want to simulate the analog deck running at a different speed, we don't want to preserve pitch or tempo.

The really big complication comes if an analog deck runs at varying speeds during recording or playback. Now that's a challenge to fix!
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